1. Field of the Invention
The present invention relates to an organic electroluminescent element, a method for producing an organic electroluminescent element, a display, and a lighting device.
2. Description of Related Art
An organic electroluminescent element (hereinafter, referring to as an “organic EL element”) is a thin film type of all-solid element formed of layers of an organic thin film (e.g., a single layer member or a multi layer member) containing an organic luminescent substance provided between a cathode and an anode. When a voltage is applied to such an organic EL element, electrons are injected from a cathode into an organic thin film layer, and holes are injected from an anode into the layer. Then, holes and electrons are recombined in a luminescent layer (i.e., an organic luminescent substance-containing layer) thereby generating excitons.
An organic EL element is a luminescent element using light emission (i.e., fluorescence and phosphorescence) of the excitons. Therefore, technologically, an organic EL element is expected to be applied to a next-generation flat display and lighting apparatus.
Further, researchers of the Princeton University reported an organic EL element using phosphorescent luminescence out of a triplet excitation state, with realizing about 4-times higher luminescent efficiency in principle than an organic EL element using fluorescent luminescence. After the report was published, development has started focusing on materials emitting phosphorescence at a room temperature, and the research and development have been extensively carried out focusing on layer structures of light emitting elements and electrodes.
As described above, a phosphorescent luminescence system has a high potentiality. However, greatly different from an organic EL device using fluorescent luminescence, an organic EL device using phosphorescent luminescence has important technological issues how to control a center position of luminescence, especially how to recombine holes and electrons inside a luminescent layer, and how to stably emit light, in order to improve the luminescent efficiency and lifetime of the device.
Hereby, mixed layers including a phosphorescent luminescence compound acting as a luminescent dopant, and a host compound are generally used in luminescent layers.
Meanwhile, in view of materials, there are high expectations for creating novel materials having improved performance of element. For example, WO2011/019156 and WO2010/083359 disclose specific triazine compounds and specific condensed ring aromatic heterocyclic compounds both of which are used for a host compound of a phosphorescent luminescence compound.
When an organic EL element used those specific compounds disclosed in WO2011/019156 and WO2010/083359 as a host compound, the luminescent efficiency of the element was mostly improved. However, remarkable decrease in luminescent intensity was observed, after the organic EL element deposited with those compounds was stored at a high temperature. Further, it was revealed that the luminescent lifetime of the element when stored at a high temperature became shorter than the element when stored at a room temperature. Moreover, there is room for further improving the luminescent efficiency.